Thermal insulation and thermally insulated device

ABSTRACT

A load supporting thermal insulator comprising a flexible casing that is filled with compacted material in the form of finely divided particles. The compacted material exerts a pressure of at least 1 lb. per square inch over the inner surface of the casing and has a coefficient of thermal conductivity less than that of the interstitial gas between the particles under the same conditions of temperature and pressure.

United States Patent Pogorski 5599 99 L LMwWWLWM NWW ZZWZW 0 066 0 0 3 22 2 ""20 21 mm m mmlm m u n u u u H n u u n u "U u n n m n m "M m "mma meta m M m u 8 "r U nm s "he numm Bk a mm mmwmwm xsmsxocho 8 1 7778 emmmssz 999999999 uwuwuuwuu 766 34 wwwmqlosszs w 720 2997 365WW67 6 1 233333232 n o 0 MW E o .m C m a, W m wmw DE 0 n m T mm 9 u 8 NM m fl o m a m A e Mm 0 P@ mmn 7 AU a 9 s .n LP UN 0 ma 7 S omd 2 NY 0 m L n u a 0 LL mm Gnc N m m EE m .m m H v s H TT h A F 1 1 M n a n v n 21 1 Appl. No.: 93,365

Primary Examiner-Robert F. Burnett Assistant ExaminerGeorge W. Moxon, ll Attorney-Gordon Waldie Related US. Application Data [63] Continuation-impart of Ser. No. 765,471, Oct.

7, 1968, abandoned. ABSTRACT A load supporting thermal insulator comprising a flex- [52] US. Cl.....................220/9 C, 138/149, 161/162 5 In. CL 333, 5 1 3 5 1 1/16 3 5 23/02 ible casing that is filled with compacted material in the 58 Field at Search ....16l/l62, 165; 220/9 A, 9 LG, form finely divided particles- The compacted 220/9 C, 9 G; 2|5/|2 13; 62/529; 138/149 material exerts a pressure of at least 1 lb. per square inch over the inner surface of the casing and has a [56] References Cited coeificient of thennal conductivity less than that of the interstitial gas between the particles under the same conditions of temperature and pressure.

UNITED STATES PATENTS 3,361,284 1/1968 Luka et al.... ...220/10 3 Claim, 6 Drawing Figures PATENIEBucIa m2 3.695.483-

SHEET 1 0F 2 Elil (PRlOR ART) l NVEN TOR.

LOU s A. POGORSKI PATENTED 3 m SHEEI 2 BF 2 INVENTOR. LOUIS A. POGORSKI THERMAL INSULATION AND THERMALLY INSULATED DEVICE This is a continuation-impart application of application Ser. No. 765,471 filed Oct. 7, 1968 now abandoned.

This invention relates to an improved type of thermally efficient, easy-to-apply, thermal insulation capable of supporting compressive loads and having a high resistance to all modes of heat transfer over a wide range of operating conditions. More particularly, this invention relates to an improved type of thermally efficient, load-supporting, thermal insulation for cryogenic, intermediate and high temperature service having a high resistance to all modes of heat transfer when placed in an environment containing gases at subatmospheric to atmospheric and above atmospheric pressures. This invention also relates to devices thermally insulated by such thermal insulation.

The main function of thermal insulation is to decrease to a minimum heat leak into or from an insulated system, whichever the case may be, in order to reduce the boil-off rate of a liquid or protect an instrument, transfer line, such as a pipe, or equipment from the effects of a heat source or a heat sink acting on the system. The increasing need for generation, storage and transfer of liquefied gases and of cryogenic fluids, as well as for the transport and operation of temperature-sensitive equipment on land, sea, under water, under ground, in the air or in space has created a de mand for economical versatile and rugged types of efficient insulations capable of withstanding a wide range of service conditions (operating temperatures from closeto absolute zero up to and over 3,000 F., operating pressures from vacuum to positive, thermal shocks, i.e., rapid, large temperature changes, compressive loads and mechanical vibration.

Although the specific requirements may vary from case to case, there is a growing demand for thermally efficient, economical, unevacuated or mildly evacuated, rugged, load-supporting thermal insulating systems having overall effective K values under actual service conditions of the order of 0.15-0.01 Btu inch/hr ft F. and lower.

Heat may be transferred from a heat source to a heat sink in three different ways: convection, conduction and radiation. The contribution of each of the above mechanisms to the overall heat transfer depends on the heat transfer medium as well as on the magnitude and level of the temperature driving force. Under certain conditions any one of the three mechanisms may become controlling, while, under other conditions, the contribution of two or of all the three modes of heat transfer may be significant.

Thermal insulation is made in general of a material having a low coefficient of thermal conductivity (K) and/or of materials chosen in such a way that heat transfer through the insulation by means of the three responsible mechanisms, or at least by the controlling mechanism, is reduced to a minimum. At normal temperature and pressure (NTP) the thermal conductivities of liquids generally are high, the thermal conductivities of solid thermal insulators are of the order of a few Btu inch/hr ft F. and the thermal conductivities of gases are in general an order of magnitude lower than that of solid thermal insulators being around one to a few tenths Btu inch/hr ft F. By a skillful combination of component materials having low K values, certain desirable properties can be emphasized and other undesirable ones can be suppressed.

The resultant composite thermal insulations will have better overall thermal insulating properties than those of the individual components thereof. The characteristics and limitations of various types of typical thermal insulating systems are listed in Table l and are reviewed below.

Characteristics of Various Types of Typical Thermal Insulating Systems I ll I" IV In- Thermal conductivity Pressure Practical limitasulrange of the insularange lions under actual ation tions as determined service condition type under laboratory con (Torr) ditions within the pressure range given in col.lll btu inch/hr fi* F (a) Single phase Solid type Relatively high insulation conductivity, relatively high density Supports required, effective conductivity affected by heat leak through joining walls and supports of containing vessel Gas type 0.0l

insulation (b) Multiphase Solid foams type insulations Conventional lO'-760 powder and fiber types of insulation Multilayer 0.0002- Reflecting 10 shields in vacuum type insulations a. Single Phase lnsulations Single phase insulations are in general homogeneous bodies most commonly made up of solids having low thermal conductivities. Their most serious limitations are relatively high heat conductivity and relatively high density, both limitations being due to the very nature of solid materials. Gas bodies confined between two walls at pressures ranging from below atmospheric to positive also can be considered single phase insulations and have limited applications.

Solid Phase Type lnsulations Heat transfer due to the mechanism of conduction through a single phase solid body is relatively large in all presently known insulating materials, limiting the insulating capabilities of single phase solid insulations to K values around I Btu inch/hr ft F. at around the am bient temperature level.

Gas Phase Type lnsulations Gas phase insulations are generally inferior to the multiphase types of insulations clue to the residual heat leak by all the heat transfer mechanisms and heat leak through the walls connecting the inner and outer part of the insulated system and through the supports req uired in most cases.

b. Multiphase lnsulations Multiphase insulations are composite structures generally made up of solid and gas phases. The most serious drawbacks common to all types of such insulations is their limited capacity to support loads. Other disadvantages of some multiphase insulation systems are high vacuum requirements and service temperature restrictions.

Solid Foams Type lnsulations The thermal conductivity of the mixed phase solid foams is decreased, in relation to the conductivity of the solid material thereof, by the presence of gas (second phase)-filled cells within a solid body (first phase). The conductivity of a composite system of this type can be modified to a degree by controlling the size of the cells, employing gases with low K values for filling the gas space and by the use of partial vacuum in the cells. A limitation is imposed by residual heat leak through the solid walls, which cannot be made infinitely thin for structural as well as other reasons. Thin walls, for instance, favor a diffusion of more conductive gases or vapors from the environment into the insulation, resulting, in the case of cells filled with gases at subatmospheric pressure, in equilibrium of the gas pressure within the cells with that of the surroundings. This factor tends to limit the performance of the solid-foams type of insulations. In practice a compromise has to be reached between the strength and the overall conductivity of the insulation. The working K values of the solid-foams insulations generally are limited to the range attainable at around atmospheric pressure (around 0.2 Btu inch/hr ft F.

Conventional Powder Or Fiber Types Of insulation The concept of using powders or loose fibers as the thermal insulating media permits a further reduction in heat conductivity to a level unattainable by the foam type insulations. in this case, continuity of heat transfer by the mechanism of direct heat conduction through a solid wall, which, in the case of solid foams could be decreased but not entirely eliminated, is almost completely interrupted by a gas interface surrounding the solid particles. The heat leak due to direct solid conduction is reduced to a limited number of solidsolid point contacts between the particles, most of the particle surface areas being screened by the residual gas film cushions, which, under normal conditions, ad here to the solid surfaces. The contribution of the solid conduction to the overall heat leak through a powder thus is minimized, as is evidenced by the fact that the overall conductivity of powders is only slightly affected by the conductivity of the powder material.

Powders or powder mixes opaque to thermal radiation are effective in decreasing the portion of heat leak attributable to this mode of heat transfer. The contribution of residual solidconduction and radiation being small, the theoretical limits of the thermal insulating capability of powders approach, and, when certain requirements are met, surpass the insulating properties of the interstitial gas. These requirements, which are predictable from the Kinetic theory of gases, indicate that the thermal conductivity of the solid gas system may be lower than the thermal conductivity of the interstitial gas at given temperature and pressure conditions when the dimensions of the gaps between the loosely-packed solids are comparable to or lower than the molecular free path in the gas phase.

The required conditions can be met by decreasing the interstitial gas pressure, and thus increasing the length of the molecular free path, by decreasing the particle size or by decreasing both. By employing these techniques, the conductivity of powder or loose fiberfilled thermal insulating systems can be decreased to a point where radiation and residual solid conduction through the point contacts will become dominant over convection. The thermal conductivities of properly formulated evacuated powder or fiber mixes can reach an efficient low of 0.01 to 0.002 Btu inch/hr ft F. at moderately mild vacuums 10 to 10' 10 Torr).

Multilayer Type insulation Multilayer thermal insulation is an extension of the composite phase insulation concept. The powdengas interface is replaced by a solid film-gas interface. The films usually are arranged in the form of a sandwich made of alternate layers of insulating and reflecting materials. The performance of this highly efficient type of insulation depends on the type of materials used, the number and thickness of the films, the physical and chemical characteristics of the sandwich layers, the amount of surface area and type of contacts between the layers, etc., and what is highly important, on maintaining a high operating vacuum level of the order of N) to 10 Torr. At high operating vacuums, K values of the order of 0.000l Btu inch/hr ft F. are obtainable.

Reflecting Shields In Vacuum This highly efficient type of thermal insulation (K values of down to the order of 0.0001 Btu inch/hr ft F consists of a multiplicity of reflective radiation shields suspended in a vacuum. However, its practical applications are seriously limited by shield suspension problems and very high vacuum requirements 10 Torr).

Typical of prior art insulation structures is that disclosed in US. Pat. No. 3,36l,284 issued Jan. 2, 1968 to Sotiri Luka et al. This patent discloses an insulation structure in which a plurality of longitudinally-extending compartments are formed on the internal surface of a wall. The lower end of each compartment is closed and each compartment is filled with free-flowing insulating powder. The powder is not compacted in the compartments. Even if care is taken in the filling of the compartments, it is impossible to avoid later settling of the powder and the consequent formation of voids. This provides inefficient insulation as explained above. Further, the structure is not designed to be laterally load-supporting. While the structure in the patent is illustrated with as supporting a roof, this is a longitudinal support which is achieved solely because of the comparatively rigid structure provided by the attachment of the casing material to the wall.

Another typical prior art structure is that disclosed in US. Pat. No. 2,937,780 issued May 24, I960 to Sterling Beckwith. In the construction illustrated in this patent, a double-walled container is provided with a plurality of pockets defined by louvres. In each of the pockets is placed a flexible bag of insulating material. The bag of insulating material primarily is intended to form a liquid impervious barrier upon contact of the bag with liquid in the container.

The insulating material in the bags, while stated to be finely divided, is not indicated to be compacted and, indeed, there is no reason to compact the material. 80 long as upon penetration of the liquid through the material of the bag a multiplicity of minute gas pockets are formed which become filled with gas and prevent movement of liquid through the insulating material. The provision of gas pockets in the insulating material is quite consistent with an uncompacted material.

The foregoing discussion of the prior art indicates that previous types of efficient thermal insulations have one or more undesirable characteristics, e.g., inability to support heavy loads, high vacuum requirements, limited operating temperature range, etc. Inability to support loads creates the necessity for relatively highly conductive solid supports which, apart from mechanical complications in the vessel structure and construction, increase the practical effective overall conductivity of the insulated system much above the theoretical values. High vacuums are difficult and costly to obtain and maintain, while a limited service temperature range narrows the applications of an insulation. Any of these factors is serious enough to considerably affect or limit the usefulness of such previous types of insulations.

It has now been found that a very satisfactory insulator may be provided, having load supporting properties and a very low K value of the order of 0.3 to 0.01 Btu inch/hr Ft F. or lower.

Additionally, the insulator of the invention does not require but can be used in high vacuum environments and is capable of withstanding mechanical vibrations. Further, depending upon the materials chosen, the insulator can be used over a wide temperature range.

A thermal insulator constituting this invention comprises a flexible casing filled with finely divided particles having a size from A. to 2 mm. The particles are compacted together within the case to provide a coefficient of thermal conductivity through the insulator less than that of the interstitial gas between the particles under the same conditions of temperature and pres sure.

In order to achieve an insulator having satisfactory thermal efficiency with respect to all modes of thermal transfer and which additionally is load-supporting, it is essential in accordance with this invention to provide thermally insulating particles which have K values less than that of the interstitial gas and which are compacted within the casing. Only in this way is it possible to reduce the K value of the insulator to less than that of the interstitial gas between the particles. Under these latter conditions, heat lost through instertial gas is minimized.

The casing may be gas permeable or impermeable, and the insulator, while preferably in the form of a tube, may take the form of a quilted blanket or a cushion, for example. The particles may be fibers, powders or fiber-powder mixes so long as each particle has a major dimension from IOA. to 2 mm.

This invention will become more apparent from the following detailed description, taken in conjunction with the appended drawings, in which:

FIG. I is a sectional view through a typical, prior art, powder-in-vacuum, thermally insulated double wall container;

FIG. 2 is a sectional view similar to FIG. I but showing a thermally insulated device embodying the instant invention;

FIG. 3 is a sectional view along the line 33 in FIG.

FIG. 4 is a view similar to FIG. 2 showing a part of the container thereof in greater detail than in FIG. 2;

FIG. 5 is a perspective view of a roll of tubular type thermal insulation embodying this invention; and

FIG. 6 is a perspective view ofa quilted blanket type thermal insulator embodying this invention.

The container 10 shown in FIG. I is of the double wall type having an inner wall 11 and an outer wall I2. Inner wall 11 defines a compartment I3 to be insulated and which may contain, for example, a liq uified gas.

The space 14 between inner wall II and outer wall I2 is filled with a powder 15 that serves as a thermal insulator. Since loose powders cannot support heavy loads or withstand vibration without settling, solid supports 16 are provided between inner wall I] and outer wall I2, these supports serving to hold the inner and outer walls in fixed relative positions with respect to each other. The inner and outer walls of the container as well as supports 16 may be of metal or other material. The heat leak through supports 16, which, in general, is very high in relation to the heat leak through powder 15, materially reduces the effective thermal insulating capability of the system as a whole, i.e., the heat leak through supports 16 appreciably increases the thermal conductivity between walls 11 and 12. Furthermore, it is difficult and, in the case of larger vessels, almost impossible, to fill space 14 with fine powders to a desired and uniform density using conventional methods. Conventionally packed powders tend to settle with time, and the heat leak due to convection currents in the resulting voids decreases the effective efficiency of the thermal insulating system. This can be offset partially by evacuating space 14, but then wall 12 must be made heavy enough to prevent it from collapsing, and supports 16 also may have to be increased in thickness, which will increase the heat leak through supports 16. Moreover, the evacuation of large spaces filled with powders in a conventional manner is both difficult and time consuming.

Referring now to FIG. 2, there is shown a thermally insulated container embodying the instant invention. Although the container shown is like that of HO. 1 and may be used, for example, for the storage of liquid oxygen, it will be appreciated that the invention is applicable to the insulation of any device that requires thermal insulation and is particularly suited for use with double wall containers or conduits such as pipe lines, for example. Container 10 of FIG. 2, like the container of FIG. I, has an inner wall 11 and an outer wall 12, the inner wall defining a compartment 13. Disposed in the space 14 between walls 11 and 12 are three layers of thermal insulation 17 embodying the instant invention. A thermal insulator constructed in accordance with the principles of the instant invention is best shown in FIG. 4 and consists of a flexible casing 18 filled with compacted material 19. The material, which may be in powder form, in fiber form or a mixture of powders and fibers, each particle having a major dimension of 10 A. to 2 mm, is compacted into flexible casing 18 to such an extent that it exerts a pressure over the whole inner surface of the casing and to provide particle-to-particle contact so that the K value of the insulator is less than that of the interstitial gas.

Casing 18, being flexible, can be shaped and bent to follow the contours of the walls of a device to be insulated, although it will be appreciated that the thermal insulator of the invention will have greater rigidity as the pressure of the compacted material in casing 18 is increased. Casing 18 may be gas permeable or gas impermeable. The use of a gas permeable casing is particularly advantageous where the thermal insulator is to operate under partially evacuated conditions, since the permeability of the casing permits interstitial gas to be evacuated from within the casing after the casing has been placed in situ, as between walls II and 12 of container ID of FIG. 2. Of course, if casing 18 is gas permeable, it should be essentially impermeable to the passage of particulate material 19 through the casing.

Casing 18 may be formed from a wide variety of materials such as, for example, glass fibers, quartz fibers, synthetic plastic or natural fiber (e.g., cotton or silk) sheets or fabrics. The choice of material for flexible casing 18 will depend upon such considerations as the pressure the casing is required to withstand, service temperature considerations and the degree of flexibility required.

As aforementioned, compacted material 19 consists of finely divided particles. The particles may be powders, fibers or mixtures of powders and fibers. A wide variety of particulate material may be employed, although it is preferred that particles of a material having a low K value and relatively inexpensive be employed. Compacted material 19 may be quartz or glass fibers, aluminum powders (opaque to thermal radiation) or mixtures of fly ash with expanded silica to name only a few possibilities.

The size of the particles may vary widely, but will range between ID A. and 2 mm. It has been found that a mixture of both large and small particles gives the best result. The shape of the particles is not a critical consideration.

An important feature of the invention is the fact that the finely divided particulate material 19 is compacted into flexible casing 18 and exerts a pressure over the whole inner surface of the casing. One result of this is to impart to a thermal insulator embodying this invention load-supporting capability. The pressure exerted by the particulate material on the casing may vary widely but generally will be between I and 300 lbs. per square inch. It is possible, however, that pressures in excess of 300 lbs. per square inch might be employed. The upper pressure limit is dictated by the strength of the flexible casing 18 in which the particulate material is compacted and the necessity to avoid crushing of the particles, which would undesirably increase surface area contact between the particles. Another advantage of having particulate material 19 compacted into casing [8 is that this minimizes any tendency for the particulate material to settle when subjected to vibrations.

The compacted material [9 should have a coefficient of thermal conductivity that is less than that of the interstitial gas between the particles under the same con ditions of temperature and pressure. Under given conditions of temperature and pressure, this variable is dependent upon particle size and the degree of compac tion.

A thermal insulator embodying this invention may be made in a number of ways. The thermal insulator 17 of FIG. 4 is tubular in configuration and preferably is made by forcing the particulate material 19 into tubular casing 18 as the casing is formed, the degree of compaction obtained being dependent upon the rate of tube formation and the rate at which it is filled with the particulate material. After casing 18 has been so filled, it preferably is passed through a pair of rollers which further compacts the particulate material.

Referring again to FIG. 2, the space 14 between walls 11 and 12 is filled with three layers of a flexible, thermal insulator embodying this invention, the insulator being tubular in configuration and wrapped around the outer surface of wall 11. It is not necessary, of course, to employ a plurality of layers of insulation. Where a large space is to be insulated, however, it is preferred to employ a plurality of smaller layers rather than a single larger insulator, since the tendency for settling of particulate material 19 will be greater with the latter than with the former. Since the insulation can support heavy loads, it is not necessary to employ supports such as those indicated at 16 of HG. 1, whereby the heat leak contributed thereby is avoided. The three layers of thermal insulation 17 shown in FIG. 2 constitute the sole supporting member extending between the inner and outer walls of container 10 of FIG. 2. Since particulate material 19 is in compacted form, settling cannot occur, so the heat transfer characteristic of the insulation will remain unaffected by vibrations.

ln assembling the container shown in FIG. 2, insulation 17 first may be wrapped around wall 11 and suitably secured thereto by tapes, nets or the like. The insulated container then may be placed inside wall 12, the top thereof being removable for this purpose.

If desired, spacers 20 shown in FIG. 3, that may be solid or hollow, corrugated or of other configuration and fabricated of metal or other material may be disposed between insulation 17 and outer wall 12, or inner wall 11 or both to form channels between one or both walls and the insulation to allow for easy evacuation. If desired, evacuation can be facilitated further by placement of suitable spacers between adjacent layers of insulation. Where evacuation in situ is contemplated, flexible casing 18 may be gas permeable. Of course, it also would be possible for the flexible casing to be gas impermeable and for the casing to be evacuated and sealed prior to use.

The heat leak paths through flexible, pre-stressed thermal insulation embodying this invention are shown by the arrows in FIG. 4. Heat leak takes place via the walls of the casings themselves and through the casing and the compacted particulate material. In view of the former it is desirable to make casings l8 thin, say, from 0.0005 to 0.0l, and of a material having a low thermal conductivity. It also is desired to stagger the layers of insulation as shown in FIG. 4 in such a way as to increase the length of the path through the casings per se from one wall of the vessel to the other.

Casings 18 can be seamless or with a seam, standard or reinforced and formed by sealing tape, stapling or stitching. Prestressed, powder-filled, flexible, tubular insulation consisting of a fiberglass or quartz fiber fabric casing and silica base powders are extremely heat resistant and can be applied in the temperature range of from close to absolute zero to around 1,200 F. in the first case and up to in excess of 2,500 F. in the latter case.

In FIG. 5 there is shown a roll of tubular, flexible, prestressed thermal insulation 17 embodying this invention. The insulation can be formed as an endless tube that can be cut into sections of desired length upon sealing of the ends thereof. As an alternative, the insulation may be in the form ofa quilted blanket, such as is shown in FIG. 6, the quilting being effected by stitching or stapling. Those skilled in the art will apprcciate that thermal insulation embodying the instant invention may assume forms other than those illustrated in FIGS. 5 and 6. Thus, prestressed, powder or fiber-filled pancakes or cushions for vessel supports, and prestressed, powder or fiber-filled bellows or sleeves for pipe line insulation may be fabricated utilizing the principles of this invention.

By way of example, a mixture of 50 percent by volume of ash and 50 percent by volume expanded silica having a particle size of from I5OA. to 0.5 microns was compacted into a casing to withstand a pressure of I20 lbs. per square inch, the casing being of 4 thou fiberglass cloth (60 warp and S8 weft plain weave). 5O

This insulation was wrapped in 3 1-inch layers around the inner wall 11 of a double wall, liquid nitrogen con tainer of the type shown in FIG. 2 and then placed within outer wall 12. The vessel had no supports 16 and was filled with I20 lbs. of liquid nitrogen. Space 14 was evacuated to 1 Torr.

The following table shows the effective overall conductivity of the insulated system after having been sub- Jected to Various tests: Actual effective overall 0.0lil5% 0.0l i l5% (LOI t l5% It is to be understood that while a thermal insulator embodying this invention can be placed in a partially evacuated environment, this is not essential, and it may be located in an environment of air or other gas, such as nitrogen, argon, helium, carbon dioxide and freon, at pressures ranging from subatmospheric to above atmospheric.

What I claim is:

l. A thermally insulated device having an inner wall, an outer wall and load-supporting thermal insulation between said walls and contacting both of said walls, said thermal insulation comprising a flexible gaspermeable casing filled with compacted finely divided particles each having a size from ID A. to 2 mm, said particles being compacted to about I to about 300 pounds per square inch, said thermal insulation being capable of supporting a load of from about 1 to about 300 pounds per square inch and constituting the sole supporting member extending between said inner and outer walls, said device having an overall thermal conductivity between said walls of the order of 0.01 Btulhrft F./in.

2. The device of claim 1 wherein the area between said walls is maintained under a partial vacuum.

3. The device of claim 2 wherein said area between said walls has a pressure of about 1 Torr. 

1. A thermally insulated device having an inner wall, an outer wall and load-supporting thermal insulation between said walls and contacting both of said walls, said thermal insulation comprising a flexible gas-permeable casing filled with compacted finely divided particles each having a size from 10 A. to 2 mm, said particles being compacted to about 1 to about 300 pounds per square inch, said thermal insulation being capable of supporting a load of from about 1 to about 300 pounds per square inch and constituting the sole supporting member extending between said inner and outer walls, said device having an overall thermal conductivity between said walls of the order of 0.01 Btu/hr.ft2 * F./in.
 2. The device of claim 1 wherein the area between said walls is maintained under a partial vacuum.
 3. The device of claim 2 wherein said area between said walls has a pressure of about 1 Torr. 